Add the read-only mode

To prevent accidental cancellation of critical tasks when used in production, let's add a read-only mode for both termui and webui.

This is a QnA comment after PyCon APAC 2023 talk.

Start by locating the termui and webui entry points and reading how task cancellation is exposed in each interface. Define the read-only behavior for both surfaces, then verify that critical tasks cannot be accidentally cancelled while production monitoring remains usable.

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